Silk is a well described natural fiber produced by the silkworm, Bombyx mori, which has been used traditionally in the form of threads in textiles for thousands of years. This silk contains a fibrous protein termed fibroin (both heavy and light chains) that form the thread core, and glue-like proteins termed sericin that surround the fibroin fibers to cement them together. The fibroin is a highly insoluble protein containing up to 90% of the amino acids glycine, alanine and serine leading to β-pleated sheet formation in the fibers (Asakura, et al., Encylopedia of Agricultural Science, Arntzen, C. J., Ritter, E. M. Eds.; Academic Press: New York, N.Y., 1994; Vol. 4, pp 1-11).
The unique mechanical properties of reprocessed silk such as fibroin and its biocompatibility make the silk fibers especially attractive for use in biotechnological materials and medical applications. Silk provides an important set of material options for biomaterials and tissue engineering because of the impressive mechanical properties, biocompatibility and biodegradability (Altman, G. H., et al., Biomaterials 2003, 24, 401-416; Cappello, J., et al., J. Control. Release 1998, 53, 105-117; Foo, C. W. P., et al., Adv. Drug Deliver. Rev. 2002, 54, 1131-1143; Dinerman, A. A., et al., J. Control. Release 2002, 82, 277-287; Megeed, Z., et al., Adv. Drug Deliver. Rev. 2002, 54, 1075-1091; Petrini, P., et al., J. Mater. Sci-Mater. M. 2001, 12, 849-853; Altman, G. H., et al., Biomaterials 2002, 23, 4131-4141; Panilaitis, B., et al., Biomaterials 2003, 24, 3079-3085). For example, 3-dimensional porous silk scaffolds have been described for use in tissue engineering (Meinel et al., Ann Biomed Eng. 2004 January; 32(1):112-22; Nazarov, R., et al., Biomacromolecules in press). Further, regenerated silk fibroin films have been explored as oxygen- and drug-permeable membranes, supports for enzyme immobilization, and substrates for cell culture (Minoura, N., et al., Polymer 1990, 31, 265-269; Chen, J., et al., Minoura, N., Tanioka, A. 1994, 35, 2853-2856; Tsukada, M., et al., Polym. Sci. Part B Polym. Physics 1994, 32, 961-968). In addition, silk hydrogels have found numerous applications in tissue engineering, as well as in drug delivery (Megeed et al., Pharm Res. 2002 July; 19(7):954-9; Dinerman et al., J Control Release. 2002 Aug. 21; 82(2-3):277-87).
However, in order to prepare silk based materials described above, chemical agents or organic solvents, such as hexafluoroisopropanol (HFIP), have been used for cross-linking or for the processing (Li, M., et al., J. Appl. Poly. Sci. 2001, 79, 2192-2199; Min, S., et al., Sen'i Gakkaishi 1997, 54, 85-92; Nazarov, R., et al., Biomacromolecules in press). For example, HFIP is used to optimize solubility of the silk and methanol is used to induce an amorphous to β-sheet conformation transition in the fibroin, in order to generate water-stable silk structures.
The use of organic solvents in the preparation of silk fibroin materials represents a significant drawback, as organic solvents pose biocompatibility problems when the processed materials are exposed to cells in vitro or in vivo. Organic solvents can also change the properties of fibroin material. For example, the immersion of silk fibroin films in organic solvents such as methanol causes dehydration of the hydrated or swollen structure, leading to crystallization and thus, loss of solubility in water. Further, with respect to tissue engineering scaffolds, the use of organic solvents can render the silk material to be less degradable. Thus, there is a need in the art for the development of silk based materials that can be formed in the absence of chemical cross-linking and/or organic solvents.